What Are the Standard Ratings of Oil-Immersed Power Transformers?

A factory replacing a 630 kVA transformer after adding a production line may initially focus on one question: should the new unit be 800 kVA, 1000 kVA, or larger?

Capacity is important, but it is not enough to define the transformer.

Suppose the existing unit operates at 11 kV/415 V, 50 Hz, with a Dyn11 vector group, a specified impedance, and an off-circuit tap changer. A new 1000 kVA transformer designed for 10 kV/400 V, 60 Hz, or a different vector group would not be a direct replacement, even though its capacity appears suitable.

The same issue appears in utility distribution, industrial plants, renewable energy substations, mining projects, commercial facilities, pumping stations, and infrastructure systems. Two oil-immersed power transformers with the same kVA or MVA rating can have very different voltage ratios, currents, impedance values, cooling arrangements, insulation levels, losses, dimensions, and operating duties.

HONLE therefore reviews the complete rating set before confirming a transformer design. For oil-immersed distribution transformers, 35 kV transformers, and 110 kV power transformers, the core, windings, insulation, tank, bushings, cooling surfaces, tap equipment, terminals, accessories, and testing requirements must all match the actual system.

Quick Answer

There is no single worldwide list of standard ratings for all oil-immersed power transformers.

Transformer capacity is expressed in kVA or MVA, but the commonly available capacity steps depend on the transformer type, voltage class, national practice, utility specification, efficiency requirement, cooling arrangement, applicable standard, and manufacturing platform.

A complete transformer rating normally includes:

• Rated capacity
• Primary and secondary voltages
• Rated currents
• Frequency
• Number of phases
• Percentage impedance
• Vector group
• Tap range
• Cooling method
• Temperature rise
• Insulation level
• No-load and load losses
• Site conditions
• Applicable standards

Common capacity and voltage values can guide an inquiry. The final transformer must still match the single-line diagram, actual load profile, fault level, grounding method, installation environment, future expansion, and required accessories.

Capacity Is Only the Starting Point

Oil-immersed power transformers are rated in apparent power because the windings, insulation, bushings, terminals, and cooling system must carry voltage and current regardless of the load power factor.

One kilovolt-ampere equals 1000 volt-amperes:

1 kVA = 1000 VA

One megavolt-ampere equals 1000 kVA:

1 MVA = 1000 kVA

Distribution transformers are commonly described in kVA. Larger industrial and substation power transformers are usually described in MVA.

The initial apparent-power requirement can be estimated from active power and power factor:

Required kVA = kW ÷ Power Factor

For example, a plant with a maximum active-power demand of 1000 kW at a power factor of 0.80 requires:

1000 ÷ 0.80 = 1250 kVA

This result is only the beginning of power transformer selection. It does not yet include:

• Motor starting current
• Simultaneous load peaks
• Harmonic current
• Load imbalance
• High ambient temperature
• High altitude
• Future expansion
• Maintenance redundancy
• Planned overload
• Emergency operating requirements

A cold-storage facility may have a moderate average load but experience a much higher short-term demand when compressors, pumps, condenser fans, and defrost heaters operate together. A metal-processing plant may have a similar average demand but require a different transformer because of welders, furnaces, rectifiers, or large motors.

HONLE therefore asks for the load profile rather than relying only on the total connected kW. Maximum demand, diversity, power factor, starting method, duty cycle, harmonics, redundancy, and expected growth all influence the final capacity.

Table 1. Main Ratings of an Oil-Immersed Power Transformer

Standard Steps Depend on the Transformer Class

The word “standard” should not be interpreted as one universal list that applies to every country and every project.

Common capacity steps are influenced by:

• National voltage systems
• Utility specifications
• Local efficiency rules
• Applicable IEC or IEEE requirements
• Transformer construction
• Cooling method
• Frequency
• Loss limits
• Manufacturer design platforms

A capacity that is commonly available for a 10 kV distribution transformer may not be a standard design for a 35 kV industrial transformer.

Distribution Ratings Follow Preferred Series

Common market examples for three-phase oil-immersed distribution transformers may include:

30, 50, 63, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, and 2500 kVA.

These values are useful for preliminary planning, but they do not mean that every capacity is automatically available with every voltage ratio, impedance, loss level, vector group, tap range, or terminal arrangement.

For example, a 1000 kVA transformer might be required as:

• 10 kV/400 V
• 11 kV/400 V
• 11 kV/415 V
• 15 kV/415 V
• 20 kV/400 V
• 22 kV/415 V
• 33 kV/400 V

Although the capacity remains 1000 kVA, the winding turns, conductor current, insulation, bushings, clearances, tank dimensions, losses, and terminal design can change significantly.

HONLE first checks whether the required combination matches an established distribution-transformer platform. If the voltage ratio, impedance, losses, site conditions, or accessories differ from the existing platform, the technical proposal identifies the required project-specific changes.

Larger Units Need Project Engineering

At 35 kV and 110 kV voltage classes, transformer capacities commonly move into several MVA or tens of MVA.

The MVA rating still does not fully define the transformer.

A 20 MVA industrial transformer and a 20 MVA utility transformer may differ in:

• Primary and secondary voltages
• Number of windings
• Percentage impedance
• Vector group
• Neutral insulation
• Tap-changing method
• Loss guarantees
• Cooling stages
• Overload duty
• Bushing arrangement
• Protection accessories
• Transport dimensions
• Terminal orientation
• Short-circuit requirements

Medium and large power transformers should therefore be confirmed through an approved technical data sheet rather than selected from capacity alone.

Voltage Ratio Must Match the System

The primary voltage is the rated voltage of the winding connected to the incoming network. The secondary voltage is the rated voltage supplied to the downstream system.

Both values must follow the single-line diagram.

Primary Voltage Must Match the Supply

Common nominal network examples include:

• 10 kV
• 11 kV
• 15 kV
• 20 kV
• 22 kV
• 33 kV
• 35 kV
• 110 kV

These values are not interchangeable.

A transformer designed for 10 kV should not be connected directly to an 11 kV network unless the winding, tap arrangement, insulation, flux density, and guaranteed performance were designed for that operating condition.

A complete inquiry should distinguish between:

• Nominal network voltage
• Rated winding voltage
• Highest voltage for equipment
• Normal operating range
• Temporary voltage variation
• Neutral grounding method
• Required insulation withstand level

This distinction is particularly important for 35 kV and 110 kV power transformers. At these voltage levels, bushing class, impulse withstand, external clearances, neutral insulation, electric-field control, and surge-protection coordination become major design inputs.

Secondary Voltage Must Suit the Load

Common low-voltage secondary examples include:

• 380 V
• 400 V
• 415 V
• 440 V
• 480 V
• 690 V

The correct value depends on the downstream switchgear, motors, drives, buildings, production equipment, and local voltage system.

A 400 V secondary should not automatically replace a 415 V transformer. The difference affects:

• Rated current
• Motor terminal voltage
• Switchgear selection
• Cable and busbar current
• Protection settings
• Voltage-drop calculations
• Transformer winding design

The voltage at the power transformer terminals may also differ from the voltage received by the load. Cable length, busbar impedance, motor starting, load variation, transformer impedance, and tap position all influence the final utilization voltage.

For long low-voltage feeders or large motor loads, HONLE reviews the transformer voltage together with the expected feeder voltage drop and starting conditions.

Rated Current Changes with Voltage

For a three-phase transformer, rated line current is calculated as:

I = S ÷ (√3 × V)

Where:

• I is the line current in amperes
• S is three-phase apparent power in volt-amperes
• V is line-to-line voltage in volts

For a single-phase transformer:

I = S ÷ V

A 1000 kVA three-phase transformer rated at 10 kV/400 V has approximate rated currents of:

• 57.7 A on the 10 kV side
• 1443 A on the 400 V side

If the secondary voltage changes to 415 V, the approximate low-voltage current becomes 1391 A.

The voltage difference may appear small, but it changes:

• Winding conductor requirements
• Low-voltage terminals
• Bushings
• Flexible connections
• Cable or busbar size
• Switchgear current rating
• Protection settings
• Temperature rise

Table 2. Rated-Current Examples for Three-Phase Transformers

These values are nominal calculations. The final winding, terminal, bushing, cable, busbar, switchgear, and protection selections must follow the approved transformer data.

Impedance Controls Fault Current and Voltage Drop

Percentage impedance is one of the most important transformer ratings because it influences both normal operation and short-circuit performance.

Transformer impedance affects:

• Available short-circuit current
• Voltage regulation
• Voltage drop under load
• Motor-starting voltage
• Protection coordination
• Switchgear fault rating
• Cable fault duty
• Load sharing between parallel transformers

A lower impedance usually reduces internal voltage drop but permits a higher short-circuit current.

A higher impedance can limit fault current but may create a larger voltage drop during motor starting or heavy loading.

Neither value is automatically better. The correct impedance must fit the complete electrical system.

Table 3. How Impedance Changes System Performance

For replacement projects, HONLE checks the proposed impedance against:

• Existing switchgear fault withstand
• Utility fault level
• Protection settings
• Motor-starting requirements
• Existing transformer test data
• Parallel-operation conditions

Impedance should be confirmed before winding design is released. It is directly related to winding geometry and leakage reactance, so changing it late can require substantial redesign.

Vector Group Must Match the Network

The vector group identifies:

• High-voltage winding connection
• Low-voltage winding connection
• Whether a neutral is available
• Phase displacement between the windings

The vector group influences:

• Grounding
• Zero-sequence current
• Triplen harmonics
• Phase displacement
• Protection schemes
• Neutral loading
• Parallel operation
• Terminal phase sequence

Two transformers with the same capacity and voltage ratio cannot necessarily operate together if their vector groups are different.

For replacement or parallel-operation projects, the inquiry should include:

• Existing vector group
• Phase sequence
• Neutral arrangement
• Grounding method
• Protection diagram
• Parallel-operation requirement

HONLE carries the approved vector group through the technical data sheet, connection drawing, winding design, terminal marking, nameplate, and test report. Keeping these documents consistent reduces the risk of incorrect connection during installation.

Tap Range Must Match Voltage Variation

Transformer taps change the effective winding turns so the voltage ratio can be adjusted.

A familiar example is:

±5% in 2.5% steps

This arrangement may provide:

• +5%
• +2.5%
• Principal tap
• −2.5%
• −5%

However, the tap range, step size, tapped winding, principal position, and operating method are project choices.

Off-Circuit Taps Need Isolation

An off-circuit tap changer can only be operated after the transformer has been de-energized, isolated, and made safe.

It is generally suitable when:

• Supply-voltage changes are infrequent
• Manual adjustment is acceptable
• Continuous voltage regulation is unnecessary

The required tap sequence should be confirmed before manufacturing.

On-Load Taps Support Live Adjustment

An on-load tap changer can change tap position while the transformer remains energized under its designed operating conditions.

It may be needed when:

• Supply voltage varies regularly
• Load conditions change substantially
• Automatic voltage regulation is required
• Remote tap-position control is needed

An on-load tap changer also requires coordination of:

• Rated current
• Tap range
• Step voltage
• Drive mechanism
• Control cabinet
• Auxiliary supply
• Automatic voltage-control logic
• Position indication
• Alarms and interlocks
• Maintenance access

For 35 kV and 110 kV projects, the tap-changer arrangement can affect winding design, tank layout, control wiring, testing, and manufacturing schedule. HONLE therefore confirms it during the technical-review stage rather than treating it as a late accessory choice.

Cooling Stages Define Available Capacity

The rating of an oil-immersed transformer depends on how effectively heat moves from the windings and core through the insulating oil to the tank, radiators, and surrounding air.

Cooling performance is influenced by:

• Internal oil paths
• Winding construction
• Tank surface area
• Radiator size
• Fan arrangement
• Ambient temperature
• Installation clearance
• Air circulation around the power transformer

ONAN Uses Natural Cooling

ONAN means:

Oil Natural, Air Natural

Oil circulates through natural convection, while heat leaves the tank or radiators through natural airflow.

ONAN cooling does not depend on fan operation for the declared ONAN rating. It is widely used for oil-immersed distribution transformers because it provides a comparatively simple cooling arrangement with fewer moving components.

Natural cooling still requires suitable installation conditions. Blocked cooling surfaces, limited clearances, poor transformer-room ventilation, direct solar exposure, or recirculating hot air can reduce heat dissipation.

For HONLE distribution transformer projects, the tank structure, cooling surface, oil volume, winding losses, and specified temperature rise are coordinated with the declared ONAN capacity.

ONAF Uses Forced Air

ONAF means:

Oil Natural, Air Forced

Oil circulation remains natural, while fans force air across the radiator surfaces.

A transformer may have separate ratings such as:

• 20 MVA ONAN
• 25 MVA ONAF

The higher rating depends on the required fans and auxiliary power being available.

The project should therefore define:

• Number of fan groups
• Fan-control method
• Control voltage
• Alarm settings
• Fan redundancy
• Loading allowed after fan failure
• Maintenance access
• Auxiliary-supply reliability

The highest nameplate capacity should not be treated as continuously available when the cooling equipment required for that stage is out of service.

Table 4. Cooling Ratings and Buyer Checks

Temperature Rise Limits Thermal Stress

Temperature rise is the difference between a specified transformer temperature and the reference ambient temperature under defined operating conditions.

Relevant thermal values may include:

• Average winding temperature rise
• Top-oil temperature rise
• Hot-spot temperature
• Oil temperature
• Ambient temperature

These values are related but not identical.

A power transformer can remain below its nominal kVA rating and still operate at an excessive temperature when:

• Ambient temperature exceeds the design basis
• Room ventilation is inadequate
• Cooling fans fail
• Harmonic current increases losses
• Radiators are blocked
• Oil circulation is restricted
• Loading is severely unbalanced
• The transformer operates repeatedly above its natural-cooling rating

When comparing technical proposals, buyers should confirm whether the stated temperature-rise value is:

• A contractual guarantee
• Based on ONAN or ONAF operation
• Based on the declared ambient temperature
• Applicable at the principal tap
• Verified by an agreed test

HONLE uses the project ambient, load, losses, cooling arrangement, and specified temperature-rise limit to determine the required winding, oil, tank, and radiator design.

Insulation Must Match the Voltage Class

The transformer insulation system must withstand the electrical stresses associated with the network.

The required insulation level depends on:

• Rated winding voltage
• Highest system voltage
• Grounding method
• Neutral insulation
• Lightning exposure
• Switching surges
• Surge-arrester coordination
• External clearances
• Bushing class
• Pollution level
• Applicable standard

Nominal voltage alone is not sufficient to define the insulation design.

For 35 kV and 110 kV power transformers, insulation coordination can affect:

• Winding insulation
• Lead arrangement
• Bushing selection
• Clearances between live parts
• Neutral-end design
• Tank clearances
• Electric-field control
• Surge protection
• Dielectric testing

The required withstand levels should appear consistently in the purchase specification, technical proposal, approved drawings, test schedule, test report, and nameplate where applicable.

HONLE confirms these values before finalizing the winding, bushing, internal lead, and clearance design.

Losses Affect Operating Cost

Two power transformers with the same capacity, voltage ratio, and impedance may still have different operating costs because their losses are different.

No-Load Loss Continues While Energized

No-load loss is produced mainly by energizing the transformer core.

It is present whenever the transformer is energized, even when the connected load is very low.

No-load performance is influenced by:

• Core material
• Flux density
• Core-joint structure
• Lamination processing
• Cutting accuracy
• Assembly quality
• Supply voltage
• Frequency

For HONLE S11 distribution transformer projects, the core material, magnetic design, cutting process, joint arrangement, and assembly control are matched to the agreed no-load loss, no-load current, and sound requirements.

This is particularly important for transformers that remain energized throughout the year, because a small difference in no-load loss continues to accumulate during both loaded and lightly loaded periods.

Load Loss Increases with Current

Load loss is produced mainly while current flows through the windings and other conductive components.

It includes winding resistance loss and additional stray losses.

Load loss is influenced by:

• Winding conductor
• Current density
• Winding temperature
• Lead arrangement
• Leakage magnetic field
• Tank and structural components
• Harmonic current

Load loss becomes particularly important for:

• Highly loaded industrial transformers
• Utility distribution networks
• Data centers
• Pumping stations
• Mining operations
• Renewable energy substations
• Continuously operating production facilities

HONLE confirms no-load and load-loss guarantees under the agreed reference conditions. Buyers can then evaluate both purchase cost and expected lifetime energy consumption.

Total Cost Includes Both Losses

The lowest purchase price does not always produce the lowest operating cost.

A complete evaluation may include:

• Transformer purchase price
• No-load loss
• Load loss
• Expected loading profile
• Electricity price
• Planned service life
• Cooling auxiliary consumption
• Maintenance requirements
• Required efficiency level

The RFQ should identify any mandatory loss or efficiency limits so the core and winding design can be matched to the project from the beginning.

Harmonic Loads Need Additional Review

Power transformers supplying nonlinear loads may carry substantial harmonic current.

Typical nonlinear loads include:

• Variable frequency drives
• Rectifiers
• Uninterruptible power supplies
• Battery chargers
• Inverters
• Welding equipment
• Electronic power supplies
• Certain furnace and traction loads

Harmonic current can increase:

• Winding eddy-current loss
• Stray loss
• Neutral current
• Local heating
• Temperature rise
• Vibration and sound

A power transformer for a harmonic-rich system should not be selected from total kW or kVA alone.

The inquiry should include available information such as:

• Load type
• Number and rating of converters
• Expected harmonic spectrum
• Total harmonic distortion
• Neutral-loading conditions
• Duty cycle
• Existing power-quality study

HONLE can then review whether the established thermal design is suitable or whether the conductor, winding arrangement, cooling, shielding, or temperature-monitoring configuration requires adjustment.

Frequency Must Match the Design

Power transformers are designed for a declared frequency, commonly 50 Hz or 60 Hz.

Frequency affects:

• Magnetic flux
• Core size
• No-load current
• Core loss
• Heating
• Sound

A transformer designed only for 60 Hz should not automatically be operated at 50 Hz at the same voltage. Lower frequency at unchanged voltage increases the volts-per-hertz ratio and can raise magnetic flux beyond the intended design level.

A transformer can be designed for both 50 Hz and 60 Hz when this requirement is stated in advance, but dual-frequency operation should not be assumed.

The frequency must remain consistent across the technical data sheet, nameplate, drawings, purchase order, and test documents.

Site Conditions Can Reduce Capacity

Transformer ratings apply under declared service conditions.

When the actual installation differs from the design basis, thermal performance, external insulation, or mechanical requirements may change.

Table 5. Site Conditions That Require a Rating Review

A transformer installed at high altitude in a hot environment may require a different cooling or insulation arrangement from the same-capacity transformer installed at sea level in a moderate climate.

HONLE therefore confirms the installation environment before releasing the final design.

Overload Needs a Thermal Assessment

There is no universal rule stating that every oil-immersed transformer can carry the same overload percentage for the same period.

Overload capability depends on:

• Initial winding temperature
• Initial top-oil temperature
• Previous loading
• Ambient temperature
• Cooling availability
• Insulation condition
• Hot-spot limit
• Load duration
• Tap position
• Harmonic content
• Auxiliary-equipment ratings
• Transformer thermal design

Emergency loading may sometimes be acceptable under a documented operating plan, but it can increase:

• Insulation ageing
• Oil temperature
• Pressure
• Gas generation
• Bushing stress
• Tap-changer stress
• Cooling-equipment duty
• Failure risk

Repeated daily overload is not the same as an exceptional emergency condition.

When a transformer is expected to operate regularly above its natural-cooling rating, the required capacity, ONAF stage, fan control, auxiliary supply, monitoring, protection, and maintenance arrangements should be included in the original specification.

Parallel Units Need Matching Ratings

Two power transformers should not be connected in parallel simply because their capacities and voltage ratios appear similar.

Parallel operation requires careful compatibility of:

• Voltage ratio
• Tap position
• Vector group
• Phase sequence
• Phase displacement
• Percentage impedance
• Impedance angle
• Frequency
• Neutral arrangement
• Grounding method
• Rated capacity

A mismatch can result in:

• Circulating current
• Unequal load sharing
• Overloading of one transformer
• Excessive heating
• Protection problems
• Incorrect phase relationships

When a new HONLE power transformer will operate in parallel with an existing unit, the buyer should provide:

• Clear nameplate photographs
• Existing test report
• Measured impedance where available
• Vector group
• Tap data
• Connection diagram
• Current operating tap
• Loading records
• Protection settings
• Phase-sequence information

The more closely the new transformer must match the existing unit, the more important the original test data becomes.

Replacement Requires Mechanical Checks

An existing nameplate is an essential starting point, but it may not contain every detail required for a direct replacement.

The replacement design should also check:

• Overall dimensions
• Wheel gauge
• Foundation layout
• Lifting points
• Jacking pads
• Terminal positions
• Cable-box dimensions
• Busbar alignment
• Bushing height
• Neutral location
• Tank arrangement
• Radiator layout
• Control-cabinet position
• Existing accessories
• Transport route
• Maximum shipping dimensions
• Maximum shipping mass

A transformer can be electrically compatible but still fail to fit the existing foundation, cable route, bus duct, enclosure, or access opening.

For replacement projects, HONLE reviews the existing general-arrangement drawing together with the nameplate and electrical data.

Documents Must Match the Transformer

The transformer nameplate provides a compact summary of the approved design.

Depending on the applicable standard and project specification, it may include:

• Manufacturer
• Serial number
• Year of manufacture
• Applicable standard
• Rated capacity
• Cooling-stage ratings
• Frequency
• Number of phases
• Winding voltages
• Winding currents
• Vector group
• Percentage impedance
• Tap positions
• Cooling method
• Temperature rise
• Insulation levels
• Total mass
• Insulating-oil mass
• Connection information

Before shipment, the buyer should compare the nameplate with:

• Approved technical data sheet
• General-arrangement drawing
• Connection diagram
• Terminal drawing
• Accessory schedule
• Routine test report
• Packing list

Capacity, voltage, current, impedance, vector group, tap positions, cooling stages, serial number, and accessories should remain consistent across the complete document set.

HONLE checks these documents against the approved design before release for dispatch.

Tests Must Follow the Contract

Transformer testing should follow the agreed standard and inspection and test plan.

The exact test program depends on:

• Transformer type
• Voltage class
• Capacity
• Applicable standard
• Purchaser specification
• Project risk level

The contract should distinguish between:

• Routine tests
• Type tests
• Special tests
• Existing test evidence
• Tests repeated for the project
• Witnessed tests
• Third-party inspection

Depending on the power transformer and project, the test program may include:

• Winding-resistance measurement
• Voltage-ratio measurement
• Vector-group verification
• No-load loss and current measurement
• Load loss and impedance measurement
• Applied-voltage withstand test
• Induced-voltage withstand test
• Insulation-resistance measurement
• Insulating-oil testing
• Leak or pressure testing
• Temperature-rise testing
• Lightning-impulse testing
• Sound-level measurement
• Partial-discharge measurement
• Functional checks of fans, alarms, controls, and tap equipment

Not every special or type test must be repeated for every transformer. The approved inspection plan should clearly identify the tests included in the contract and the reports supplied with the equipment.

HONLE confirms the testing scope during technical and commercial review so the quotation, production plan, and delivery documents follow the same requirements.

HONLE Matches Ratings to Operating Conditions

HONLE manufactures oil-immersed distribution transformers as well as 35 kV and 110 kV power transformers.

The engineering focus changes with the voltage class and project duty.

For an oil-immersed distribution transformer, the main considerations may include:

• Capacity platform
• Voltage ratio
• Core and winding design
• Loss requirements
• ONAN cooling
• Off-circuit tapping
• Tank construction
• Compact installation
• Low-voltage terminal current

For a 35 kV or 110 kV power transformer, additional attention may be required for:

• Insulation coordination
• Winding short-circuit strength
• Bushing selection
• Mechanical winding support
• Radiator and fan stages
• On-load tap changing
• Neutral insulation
• Monitoring and protection
• Transport limitations
• Project-specific testing

HONLE’s manufacturing role begins before production. The single-line diagram, load data, voltage ratio, impedance, vector group, tap arrangement, losses, cooling, insulation level, accessories, terminals, site conditions, standards, and test requirements are reviewed together.

Once the technical data is approved, the same rating set is carried through:

• Electromagnetic design
• Core and winding manufacture
• Insulation arrangement
• Tank and cooling design
• Bushing and accessory selection
• Drawings
• Nameplate
• Testing
• Packing and delivery documents

This complete process is more reliable than selecting a transformer from one familiar capacity value and resolving the remaining differences after manufacturing has started.

A Complete RFQ Speeds Up Quotation

A detailed RFQ helps HONLE prepare an accurate technical proposal and reduces repeated clarification after quotation.

Table 6. Information to Include in a Power Transformer RFQ

For a replacement transformer, also provide:

• Existing nameplate photograph
• Technical data sheet
• General-arrangement drawing
• Connection diagram
• Test report
• Foundation dimensions
• Terminal layout
• Existing accessory list
• Parallel-operation information
• Installation photographs

The more complete the inquiry, the easier it is to identify which requirements match an established HONLE platform and which parts need project-specific engineering.

The Complete Rating Determines Suitability

The standard rating of an oil-immersed transformer cannot be reduced to one capacity number.

A transformer may carry a familiar 630 kVA, 1000 kVA, 2500 kVA, 20 MVA, or 63 MVA rating, but its suitability still depends on:

• Voltage ratio
• Rated current
• Frequency
• Phase
• Impedance
• Vector group
• Tap arrangement
• Cooling
• Temperature rise
• Insulation level
• Losses
• Site conditions
• Load characteristics
• Testing
• Accessories
• Installation arrangement

Buyers should compare the complete guaranteed data rather than capacity alone.

With the system information confirmed, HONLE can match the project to an oil-immersed distribution, 35 kV, or 110 kV transformer platform and define the required electrical, thermal, mechanical, and testing configuration before production begins.

Frequently Asked Questions

What do kVA and MVA mean on an oil-immersed transformer?

They state the apparent-power capacity under declared voltage, frequency, cooling, temperature, and service conditions. One MVA equals 1000 kVA.

Is transformer capacity the same as the connected load in kW?

No. Transformer capacity is expressed in kVA or MVA. Power factor, load peaks, motor starting, harmonics, diversity, and future growth also affect the required capacity.

Can two transformers with the same kVA rating be different?

Yes. They can have different voltage ratios, currents, impedances, vector groups, taps, losses, insulation levels, cooling arrangements, dimensions, and applications.

Are transformer capacity steps the same in every country?

No. Common steps vary with national practice, utility requirements, system voltage, efficiency rules, transformer type, and manufacturing platform.

What is the difference between rated voltage and system voltage?

Rated voltage applies to the transformer winding. System voltage describes the network. The highest voltage for equipment and normal operating range must also be confirmed.

Why does transformer impedance matter?

Impedance affects fault current, voltage drop, motor starting, protection coordination, switchgear duty, and parallel load sharing.

Can a lower-impedance transformer replace a higher-impedance unit?

Not automatically. The lower impedance may increase fault current beyond the capability of existing switchgear or alter protection coordination.

Can transformers with different vector groups operate in parallel?

Normally not. Their winding connections and phase displacement must be compatible before parallel operation is considered.

What is the difference between ONAN and ONAF?

ONAN uses natural oil circulation and natural airflow. ONAF adds forced airflow from fans while oil circulation remains natural.

Can the ONAF rating be used without operating fans?

No. When the required fans are unavailable, loading should remain within the capacity permitted for the available cooling stage.

Does high ambient temperature reduce transformer capacity?

It can. Higher ambient temperature reduces thermal margin and may cause oil or winding temperature limits to be reached at a lower load.

Does altitude affect oil-immersed power transformers?

Yes. High altitude can reduce air-cooling effectiveness and external dielectric strength. The installation altitude should be stated in the RFQ.

Can an oil-immersed transformer operate above its nameplate rating?

Only after a project-specific thermal assessment. Previous loading, ambient temperature, cooling status, hot-spot temperature, load duration, and insulation condition must be considered.

Does insulating-oil type affect the transformer rating?

It can affect cooling, insulation, fire planning, environmental requirements, and thermal performance. The required liquid should be confirmed before design.

Do VFDs and inverters affect transformer selection?

They can. Harmonic currents may increase winding and stray losses, so the load type and available harmonic data should be provided.

Can a 60 Hz transformer operate on a 50 Hz system?

Not automatically. Operating at a lower frequency without suitable voltage adjustment may increase magnetic flux and heating.

Why are no-load and load losses listed separately?

No-load loss occurs while the transformer is energized. Load loss increases as the transformer carries current. Both affect operating cost and temperature.

What information is needed for a direct replacement?

Provide the nameplate, technical data, drawings, test report, foundation dimensions, terminal layout, accessories, impedance, and vector group.

What should appear on the transformer nameplate?

Depending on the applicable standard, it may include capacity, voltages, currents, frequency, phase, vector group, impedance, taps, cooling, insulation levels, temperature rise, mass, and serial number.

Are routine tests and type tests the same?

No. Routine tests are performed on each transformer as required. Type tests verify selected design characteristics and are repeated only when required by the contract or design conditions.

Should buyers request a guaranteed technical data sheet?

Yes. It records the approved capacity, voltage ratio, impedance, losses, cooling, insulation, temperature rise, taps, accessories, and other requirements before manufacturing.